Denny, if you do the math, the conversion should have given you a 13.6% increase actual speed versus speedo reading making the assumption that your speedo was dead accurate before the change. Since you are now benchmarking it off GPS, it is hard to tell what your true increase was versus the speedo unless you have the same GPS/speedo data points from before the installation.
The fix is simple as Joe points out. I bought an adapter from Jim Carter and am very happy with it. His website has instructions of what and how to measure to get your adapter custom made for your application. As I recall, the unit is calibrated for accuracy at 50MPH knowing that there will be some deviation above and below that. Other companies also offer an adapter. Here is a link to Jim Carter's.
http://www.oldchevytrucks.com/cart/product.asp?prodid=SP249&i=|speedometer%20adapter||||
Matt
